Fuel cell power systems convert a fuel and an oxidant to electricity. One fuel cell power system type of keen interest employs use of a proton exchange membrane (hereinafter “PEM”) to catalytically facilitate reaction of fuels (such as hydrogen) and oxidants (such as air/oxygen) into electricity. The PEM is a solid polymer electrolyte that facilitates transfer of protons from the anode to the cathode in each individual fuel cell of the stack of fuel cells normally deployed in a fuel cell power system.
In a typical fuel cell assembly (fuel cell stack) within a fuel cell power system, individual fuel cells have flow fields with inlets to fluid manifolds; these collectively provide channels for the various reactant and cooling fluids reacted in the fuel cell stack to flow into each cell. Gas diffusion assemblies then provide a final fluid distribution to further disperse reactant fluids from the flow field space to the reactive anode and cathode.
Coordinated shutdown is one factor in effective operation of a fuel cell stack or set of fuel cell stacks. In this regard, hydrogen is a substantive component of the fuel fed to the reactive anodes of the fuel cell stack, and, upon shutdown, a working inventory of hydrogen-containing fuel is present in the fuel reactant channels of the fuel cell stack. While not inherently flammable as relatively high concentration hydrogen, the residual fuel in the stack is subject to potential mixing with oxygen in air, especially if the fuel cell is inoperable for a period of time. Such mixing of oxygen has the potential to create a potentially flammable mixture within the fuel cell stack at a certain threshold composition.
Two general shutdown strategies or modes represent the scope of shutdown approaches of a fuel cell system: a “normal” shutdown mode and a “rapid” shutdown mode. “Normal” shutdown proceeds through a process of (a) disconnecting the load, (b) consuming excess hydrogen, and (c) cooling the system in a manner to minimize internal stresses induced from thermal change. With a “rapid” shutdown, conditions are present such as a malfunction or error detection which require the fuel cell to be shut down in a manner which does not enable the time needed for consuming excess hydrogen. When a “rapid” shutdown is implemented, the residual fuel is handled by venting reformate or hydrogen to the atmosphere.
There are typically two venting solenoid valves for fuel reactant in a conventional fuel cell stack of a fuel cell power system: a first valve at the outlet of the fuel processor generating the fuel and a second valve at the outlet of the anode manifolds of the stack. During normal operation, these venting valves are closed to prevent hydrogen leakage and/or discharge to the atmosphere. Conventional fuel cell power systems not having a fuel processor typically use at least one vent valve on the stack anode. During a “rapid” shutdown, each venting valve opens to vent hydrogen to the atmosphere. This approach rapidly removes potential and thermal energy from the system and vents residual hydrogen within the fuel cell.
These vent valves have traditionally been provided as normally-open valves in which the valve is biased open via spring returns. As should be apparent, in the powered-down state, these valves are open and therefore allow fluid flow into and out of the anode manifolds. The fail-open valve is deployed so that the system vents fuel out the combustible vent even during a full loss of system power (usually the most dramatic “rapid” shutdown situation). As should also be apparent, a normally-open vent valve remains open after the system has shutdown in the continued absence of any electrical power. Such as when the system is turned off.
One disadvantage, however, in using normally-open vent valves is that, in some particular shutdown instances where the vent valves remain open, some residual hydrogen remains in the fuel cell manifolds. When a long downtime ensues, it is possible for air to flow through the vent over time until a combustible mixture occurs in some part of the system. It is also possible for a combustible mixture to evolve when power failure occurs after a “normal” shutdown executed without a purge (e.g., a “normal shutdown” initiated under controlled conditions which deteriorates into a “rapid” shutdown due to power loss).
Another disadvantage in using normally-open vent valves is that catalyst deactivation may ensue after shutdown of the fuel cell and loss of power. In this regard, a number of catalysts deactivate in the presence of air, especially immediately after shutdown when the catalyst is still thermally “hot”. One approach to handling this concern is to use an air tolerant catalyst; but this approach severely hampers the choice of possible catalysts. Another approach is to provide a purge mechanism whereby hydrogen is purged from the system with an inert gas such as nitrogen to provide a nitrogen blanket over the active catalyst elements. Such alternatives define a more expensive design than is achievable with a catalyst not as air tolerant.
A further disadvantage in using normally-open vent valves is that, in powered-down mode, a path to the fuel processor and/or stack manifolds is available for dust and other potentially harmful elements to contaminate the internal channels and surfaces of the fuel cell. Such contamination shortens fuel cell life and also may diminish fuel cell performance in comparison to a fuel cell which is not contaminated.
An additional disadvantage in using normally-open vent valves derives from controller lockup where the control computer for the fuel cell system establishes fail-last control element positioning. In this context, the normally-open vent valves may be inappropriately sustained in a closed position.
Yet another disadvantage in using normally-open vent valves derives from the fact that all normally-open valves must be energized to remain shut—the primary operating state of vent valves. As such, these normally-open vent valves constitute a parasitic power burden on the fuel cell power system in normal real-time operation, inherently lowering overall operating system efficiency.
What is needed is a holistic approach to fuel cell venting and purging which provides coordinated shutdown of the fuel cell at low cost, protection of the catalyst after a power failure in the fuel cell power system, a basis for appropriate shutdown of the fuel cell stack and/or fuel cell stack set when conditions collectively indicate the need for such an operational event, and optimal efficiency in fuel cell power system operation. The present invention is directed to fulfilling this need.